Hydrogenation with heterogeneous catalysts

Addition of hydrogen to a multiple bond is hydrogenation. It is applicable to almost all types of multiple bonds and is of great importance in synthetic chemistry, particularly in the chemical industry. Probably the most important technical example is production of ammonia by the hydrogenation of nitrogen  

This may appear to be a simple process, but in fact it is difficult to carry out because the equilibrium is not very favorable. High pressures (150-200 atm) are required to get a reasonable conversion, and high temperatures (430-510°) are necessary to get reasonable reaction rates. A catalyst, usually iron oxide, also is required. The reaction is very important because ammonia is used in ever-increasing amounts as a fertilizer either directly or through conversion to urea or ammonium salts. 

Production of ammonia requires large quantities of hydrogen, most of which comes from the partial oxidation of hydrocarbons with water or oxygen. A simple and important example is the so-called methane-steam gas reaction, which is favorable only at very high temperatures because of the entropy effect in the formation of H2 (see Section 4-4B)  

Therefore the fertilizer industry is allied closely with the natural gas and petroleum industries, and for obvious reasons ammonia and hydrogen often are produced at the same locations. 

Alkenes and alkynes add hydrogen much more readily than does nitrogen. For example, ethene reacts rapidly and completely with hydrogen at ordinary pressures and temperatures in the presence of metal catalysts such as nickel, platinum, palladium, copper, and chromium  

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Review on stereoselective hydrogenation with heterogeneous catalysts K. Har-ada, Asymmetric Heterogeneous Catalytic Hydrogenation, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 5, Chap. 10, Academic Press, New York, 1985. 

Reduction of nitro compounds to amines is a synthetically important reaction (98) and is practiced since the birth of modern chemical industry—many aromatic amines are key intermediates in production of dyes and pesticides. However, the stoichiometric reductions using iron or alkali metal hydrogen sulfides or catalytic hydrogenations with heterogeneous catalysts leave room for improvements in selectivity, especially with reference to halonitro-derivatives. There are many homogeneous catalysts such as the rhodium carbonyls in the presence of amines or chelating diamines, or [Rus(CO)i2] in basic amine solutions that are... 

Traditionally, monoqrclic arene hydrogenation is carried out in drastic conditions with heterogeneous catalysts [9-18] such as Rh/Al203 and Raney Nickel or metal sulfides. Nevertheless, some pure homogeneous systems have been reported [19-23]. 

The use of dispersed or immobilized transition metals as catalysts for partial hydrogenation reactions of alkynes has been widely studied. Traditionally, alkyne hydrogenations for the preparation of fine chemicals and biologically active compounds were only performed with heterogeneous catalysts [80-82]. Palladium is the most selective metal catalyst for the semihydrogenation of mono-substituted acetylenes and for the transformation of alkynes to ds-alkenes. Commonly, such selectivity is due to stronger chemisorption of the triple bond on the active center. 

Other salts of formic acid have been used with good results. For example, sodium and preferably potassium formate salts have been used in a water/organic biphasic system [36, 52], or with the water-soluble catalysts discussed above. The aqueous system makes the pH much easier to control minimal COz is generated during the reaction as it is trapped as bicarbonate, and often better reaction rates are observed. The use of hydrazinium monoformate salts as hydrogen donors with heterogeneous catalysts has also been reported [53]. 

The selectivity for hydrogenation of dienes in the presence of monoolefins arises from the exceptional stability of jr-ally 1 complexes. In the case of Pt catalysts the reactions shown can compete with one another (equation 6)14. The second pathway is favored, especially when the olefin or diene must compete with excess ligands (phosphine, CO, SnCp ) for a coordination site. This is why the diene is almost completely hydrogenated before the concentration of olefin increases to the point that the olefin gains access to the catalyst. A similar phenomenon can be responsible for selectivity in hydrogenation of dienes with heterogeneous catalysts. 

The mechanisms of hydrogenation of alkenes over finely divided metals such as nickel, platinum, and so on (Section 11-2) now are understood in a general way. However, these reactions are extremely difficult to study because they occur on a metallic surface whose structure is hard to define. In contrast, the mechanisms of hydrogenation with homogeneous catalysts are known in considerable detail and provide insight into their heterogeneous counterparts. 

Compared to the hydrogenation to 5P-ketones, the hydrogenation of 3-oxo-A4 steroids to 5a-ketones is much more difficult over usual heterogeneous catalysts, except in the special cases where the presence of oxo groups may favor the formation of 5a compounds as described above. Dauben, Jr. et al. obtained 5a-cholestan-3-one from cholest-4-en-3-one via transformation into 3-ethylenedioxy A5-derivative 117 by exchange dioxolanation, followed by hydrogenation with palladium catalyst to give exclusively 3-ethylenedioxy-5a-cholestane (eq. 3.79).274 The ethylenedioxy compound may be hydrolyzed quantitatively to the 5a-ketone in the presence of acid. 

Hydrogenation of aromatic nitro compounds with heterogeneous catalysts is often the method of choice for the production of the corresponding anilines. As... 

Hydrogenations using heterogeneous catalysts usually require thermal condition above room temperature and H2 pressures higher than 10 Pa. Homogeneous catalysts are often more selective with individual reactions and make the reactions possible at lower temperatures and H2 pressures. 

The reader is referred to Pryde (86) for a more thorough discussion on the kinetics of autoxidation of phospholipids their forming metal ion, iodine, and other complexes halogen addition and their behavior during hydration, hydrogenation (with heterogeneous and homogeneous catalysts), hydrolysis and alcoholysis, hydroxylation, oxidation, radical, and other reactions. 

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